1. Field of the Invention
The present invention relates to a method and apparatus for transferring a lead strip of a paper-web from a first traveling surface (e.g., of a press roll or of a press belt) of a paper-making machine to a following section (e.g., a further wet press or drying section) of that machine. In particular, the beginning of a still wet lead strip (or xe2x80x9ctailxe2x80x9d) is transferred according to the invention. This is to facilitate the threading of the paper web into the machine.
2. Description of the Related Art
In a dewatering press section of a papermaking machine, as disclosed in FIG. 1 of U.S. Pat. No. 5,404,811, a still wet paper web 9 travels from a wire belt 8 of a forming section through three press nips 16/17, 1/2 and 2/3 and thereafter via a paper roll 59 to a further dewatering press 60; then the still wet web is transferred to a drying section (not shown). When the machine is started up or after a web break, the web is threaded into the machine in a known manner:
Initially the web (having the full web width) is running behind the third press nip downwardly and is guided by a doctor 14 into a broke pulper (not shown, positioned below the machine). The web includes a small edge strip, namely the so-called lead strip or xe2x80x9ctailxe2x80x9d, severed from the web by a water jet positioned in the forming section. This tail is now transferred across paper roll 59 and via a bottom press felt through press 60 as well as through the following drying section. Then the width of the tail is increased up to the full width of the web.
During the threading operation, the web is already running with the full machine speed which may be more then 1000 m/min, in modem high speed machines up to about 2000 m/min. Therefore, the transfer of the tail from press roll 2 to the press felt of press 60 is a very difficult step of the threading operation. Sometimes this is done manually by means of an air jet being directed onto the surface of roll 2, thusly severing the tail and forming a new beginning of the tail guided across paper roll 59 to the further press 60.
A modem high speed paper-making machine normally comprises an apparatus for carrying out this difficult step. One known apparatus of this type is disclosed in FIG. 2 of U.S. Pat. No. 5,635,030. Here again, a paper web 1 is traveling downwardly across a press roll 5 from a press nip toward a doctor 7. A blast nozzle 6 (or xe2x80x9cseparating blow pipexe2x80x9d) is provided to peel off the tail from press roll 5 and to transfer the tail to paper roll 2. A further blast nozzle 3 is arranged between the two rolls 5 and 2, which blast nozzle creates an air stream, the velocity of which is greater than the velocity of paper roll 2. Due to the Coanda-Effect, the air stream adheres to the rotating shell of paper roll 2 and guides the tail up to a stationary guide plate 9 which deflects the air stream and the tail toward subsequent press unit 8.
The method and the apparatus disclosed in the ""030 patent have some disadvantages. Among others, two blast nozzles are needed, one being positioned between the press roll 5 and the paper roll 2. Also, there is a large distance between the paper roll 2 and the infeed area of the press felt 17 of the following press 8. As a result, the tail transfer to the following press may not always be successful in a reliable manner.
In another concept, only a short distance has been provided between the paper roll and the infeed area of the following press felt. In other words, a felt roll guiding the following press felt to the following press has been arranged relatively close to the paper roll (as shown in FIG. 1 of the ""811 patent). Also, an air cushion has been created onto the infeed area by means of a series of blast nozzles. However, this design also does not always operate satisfactorily. Also, a so-called pony roll has been suggested to be arranged on the infeed area instead of an air cushion, however this is mechanically complicated and therefore not desirable.
The present invention significantly improves the transfer of a tail from a first traveling surface to a following machine section by use of a novel method and by use of an improved apparatus which operates more reliably than previous proposals.
The method requires little operator skill. Thus, inexperienced personnel are able to start the threading operation without a lot of practice.
The threading operation is easily started by a reliable transfer of the tail, even with different paper grades (e.g., different basis weight) and with different machine speeds, including extremely high speed (e.g., more than 2000 m/min).
A lead strip or xe2x80x98tailxe2x80x99b of a paper web is transferred from a first traveling surface of an element of a paper-making machine to an infeed area of a second traveling surface which guides the tail into a following machine section.
The first traveling surface is the surface of a rotatable shell of a press roll which directly contacts the still wet paper web and which is part of a web dewatering press. It may also be the surface of a press belt traveling through a press nip of a web dewatering press.
The second traveling surface is the web-carrying surface of a dewatering press felt which guides the web through a subsequent dewatering press. However, the second traveling surface may also be the surface of a subsequent press roll or of a subsequent press belt. In another embodiment of the invention, the second traveling surface is the surface of a dryer fabric which guides the web through a part of a dryer section following the press section of the paper-making machine.
The method of the present invention includes providing at least one air jet for peeling off of the tail from the first traveling surface and for transferring it across a rotating paper roll to the infeed area of the second traveling surface. More particularly, the air jet initially severs the tail running with the first traveling surface, thereby forming a new beginning of the tail which is now transferred to the second traveling surface.
An air cushion is provided on the infeed area of the second traveling surface. The air cushion is created by an air table having a plurality of tiny orifices delivering air from an air plenum to the air cushion. According to the invention, the air cushion a created by a large number of tiny orifices which connect the air plenum to the air cushion and which are distributed substantially equally on the air table. This results in a significant advantage, namely that the oncoming tail (including its new beginning) is forced to lay on the second traveling surface with only a small amount of air moving with the second traveling surface. Therefore, the tail, which is running at the high machine speed together with the second traveling surface, is reliably pressed onto the second traveling surface. In other words, the air does not bounce off the second traveling surface and the tail is not lifted with the air. Such an undesirable behavior would result from creating an air cushion by use of a series of blast nozzles which deliver too much air at a too high pressure.
Too much air is also not directed into the following machine section, e.g., into a further dewatering press. Therein the press nip (or a similar wedge-like gap) would cause the air to flow sideways and to take the tail with it. This could happen both before the beginning of the tail has arrived in the press nip and thereafter.
In summary, the method of the invention results in a very easy and reliable transfer of the tail so that the transfer does not need much operator skill. Also, the transfer works well with different paper grades and with different machine speeds including the high speed of modem paper machines.
The present invention also includes an apparatus for transferring the tail as described above. The apparatus includes a peeling jet device for providing the at least one air jet as mentioned above. The peeling jet device is arranged close to the first traveling surface (e.g., the surface of the press roll).
The apparatus further includes a conventional paper roll which guides the web between the first and second traveling surfaces. The most important element of the apparatus of the invention is a so-called air table having a large number of tiny holes which connect an air plenum with the infeed area of the second traveling surface, thereby creating an air cushion which is almost stationary on the second traveling surface. This results in the advantages described above.